Surface shape recognition sensor

ABSTRACT

A structure ( 113   b ) which includes an overhang and a support portion supporting substantially the center of the overhang, and in which the area of the support portion is smaller than the area of the overhang in the two-dimensional direction of an upper electrode ( 110   b ) is formed on the upper electrode ( 110   a ) in a region above each lower electrode  105   a  in one-to-one correspondence with the lower electrode ( 105   a ). An object of surface shape sensing, e.g., the tip of a finger ( 1602 ) touches the surface of the overhang of the structure ( 113   b ), and the support portion of the structure ( 113   b ) whose overhang is in contact with the object of sensing pushes down a portion of the upper electrode ( 110   a ) toward the lower electrode ( 105   a ), thereby deforming the upper electrode ( 110   a ).

The present patent application is a non-provisional application ofInternational Application No. PCT/JP2004/001532, filed Feb. 13, 2004.

TECHNICAL FIELD

The present invention relates to a surface shape recognition sensor usedto sense a surface shape having a fine three-dimensional pattern such asa human fingerprint or animal noseprint, and a method of fabricating thesame.

BACKGROUND ART

Along with the advance of the information-oriented society and in theenvironment of the modern society, the security technology has receiveda great deal of attention. For example, in the information-orientedsociety, a personal authentication technology for constructing a systemfor, e.g., electronic cashing is an important key. Authenticationtechnologies for preventing theft or unauthorized use of credit cardshave also been extensively researched and developed. (Reference 1:Yoshimasa Shimizu et al., “A Study on the Structure of a Smart Card withthe Function to Verify the Holder”, Technical report of IEICE OFS92-32,pp. 25–30 (1992))

There are various kinds of authentication schemes such as fingerprintauthentication and voice authentication. In particular, many fingerprintauthentication techniques have been developed so far. Fingerprintreading schemes include an optical scheme which includes an opticalsystem such as a lens and illumination, a pressure scheme using apressure sheet, and a semiconductor scheme by which a sensor is formedon a semiconductor substrate. Of these schemes, the semiconductor schemeallows easy miniaturization and generalization. An example of thesemiconductor sensor is a capacitive fingerprint sensor using the LSIfabrication technique. (Reference 2: Marco Tartagni and RobertGuerrieri, “A 390 dpi Live Fingerprint Imager Based of FeedbackCapacitive Sensing Scheme”, 1997 IEEE International Solid-State CircuitsConference, pp. 200–201 (1997)) This fingerprint sensor senses thethree-dimensional pattern of a skin by using the feedback capacitivescheme by a sensor chip in which small capacitive sensors aretwo-dimensionally arranged on an LSI.

This capacitive sensor will be described below with reference to asectional view in FIG. 14. The sensor includes a sensor electrode 1403formed on a semiconductor substrate 1401 via an interlevel dielectric1402, and a passivation film 1404 which covers the sensor electrode1403. Although not shown in FIG. 14, on the semiconductor substrate 1401below the interlevel dielectric 1402, a sensing circuit which is anintegrated circuit including a plurality of MOS transistors and aninterconnecting structure is formed. When a finger as an object offingerprint sensing touches the passivation film 1404 (sensing surface)of the sensor chip having the above arrangement, the sensor electrode1403 and the skin of the finger function as electrodes to form acapacitance.

This capacitance is sensed by the sensing circuit described above via aninterconnection (not shown) connected to the sensor electrode 1403.However, since the capacitive fingerprint sensor uses the skin as anelectrode, the built-in integrated circuit of the sensor chip iselectrostatically destroyed by the static electricity generated at thefingertip.

To prevent this electrostatic destruction of the capacitive fingerprintsensor described above, a surface shape recognition sensor including acapacitive sensor having a sectional structure as shown in FIG. 15 isproposed. The arrangement of the sensor shown in FIG. 15 will beexplained below. This sensor includes a sensor electrode 1503 formed ona semiconductor substrate 1501 via an interlevel dielectric 1502, adeformable plate-shaped moving electrode 1504 positioned at apredetermined distance from the sensor electrode 1503, and a supportmember 1505 which is formed around the sensor electrode 1503 so as to beinsulated and separated from the sensor electrode 1503, and supports themoving electrode 1504.

When a finger as an object of fingerprint sensing touches the movingelectrode 1504, the pressure from the finger deflects the movingelectrode 1504 toward the sensor electrode 1503, thereby increasing thecapacitance formed between the sensor electrode 1503 and movingelectrode 1504. This capacitance is sensed by a sensing circuit on thesemiconductor substrate 1501 via an interconnection (not shown)connected to the sensor electrode 1503. In this surface shaperecognition sensor, when the moving electrode 1504 is grounded via thesupport member 1505, the static electricity generated at the fingertipflows to ground via the support member 1505 even if the electricity isdischarged to the moving electrode 1504. This protects the built-insensing circuit incorporated below the sensor electrode 1503 from theelectrostatic destruction.

In addition to the surface shape recognition sensor shown in FIG. 15, astructure having a cubic projection 1601 as shown in FIG. 16 is alsoproposed. (Reference 3: Japanese Patent Laid-Open No. 2002-328003) Inthis structure, a force from a finger 1602 can be transmitted to themoving electrode 1504 more efficiently than in the structure shown inFIG. 15.

Unfortunately, the above conventional fingerprint sensors have theproblem that no desired high sensitivity can be obtained. For example,in the fingerprint sensor having the arrangement shown in FIG. 14, thesensitivity largely changes in accordance with the state of the fingersurface, so it is not easy to stably obtain high sensitivity. Also, inthe fingerprint sensor having the arrangement shown in FIG. 15, no largechange in upper electrode can be obtained, and this makes it impossibleto obtain high sensitivity.

Furthermore, in the structure shown in FIG. 16, the projection 1601 isreadily damaged by a force applied sideways to the moving electrode1504, e.g., a force produced by scratching, and this lowers themechanical strength. In addition, in the structure shown in FIG. 16, theprojection 1601 sinks into the finger 1602 if it is soft, and the forcedisperses in a region on the support member 1505 of the moving electrode1504, thereby lowering the sensitivity.

DISCLOSURE OF INVENTION

The present invention has been made to solve the above problems, and hasas its object to provide a surface shape recognition sensor which sensesa shape such as a fingerprint at high sensitivity, and has highmechanical strength.

A surface shape recognition sensor according to the present inventioncomprises a plurality of capacitance sensing elements comprising aplurality of lower electrodes arranged in the same plane on a substrateso as to be insulated and separated from each other, and a deformableplate-shaped upper electrode formed above the lower electrodes with apredetermined spacing between them, and made of a metal, a supportmember which is formed around the lower electrode so as to be insulatedand separated from the lower electrode, and supports the upperelectrode, and a structure formed on the upper electrode in a regionabove each of the lower electrodes in one-to-one correspondence with thelower electrode, wherein the structure comprises an overhang and asupport portion which supports substantially a center of the overhang,and an area of the support portion is smaller than an area of theoverhang in a two-dimensional direction of the upper electrode.

In the surface shape recognition sensor having the above arrangement, anobject of surface shape sensing, e.g., a fingertip touches the surfaceof the overhang of the structure, and the support of the structure whoseoverhang is in contact with the object of sensing pushes down a portionof the upper electrode toward the lower electrode, thereby deforming theupper electrode.

The above surface shape recognition sensor may also comprise a thinflexible film formed on the overhang of the structure, and extendingover a plurality of the structures, thereby inhibiting an invasion of anobject from the gap between adjacent structures. The sensor may alsocomprise a perimeter support portion formed below a perimeter of theoverhang, and made of an elastic material, thereby preventing a fall ofthe structure.

In the above surface shape recognition sensor, the substrate may be asemiconductor substrate on which an integrated circuit is formed, thelower electrode may be placed on a interlevel dielectric formed on thesemiconductor substrate, and the integrated circuit may include asensing circuit which senses a capacitance formed on the lowerelectrode.

A surface shape recognition sensor fabrication method according to thepresent invention comprises the steps of forming an interleveldielectric on a semiconductor substrate, forming a first metal film onthe interlevel dielectric, forming a first mask pattern having a firstopening portion on the first metal film, forming a first metal patternby plating on a surface of the first metal film exposed to a bottom ofthe first opening portion in the first mask pattern, removing the firstmask pattern, and forming, on the first metal film and first metalpattern, a second mask pattern having a second opening portion aroundthe first metal pattern, forming a second metal pattern thicker than thefirst metal pattern by plating on the surface of the first metal patternexposed to a bottom of the second opening portion in the second maskpattern, removing the second mask pattern, and etching away the firstmetal film by using the first metal pattern and second metal pattern asmasks, thereby forming a lower electrode made of the first metal filmand first metal pattern, and a support member made of the first metalfilm and second metal pattern, forming a first sacrificial film on theinterlevel dielectric so as to cover the lower electrode and expose anupper portion of the support member, forming an upper electrode having aplurality of third opening portions on the first sacrificial film andsupport member, selectively removing the first sacrificial film via thethird opening portions after the upper electrode is formed, forming asealing film on the upper electrode after the first sacrificial film isremoved, forming, on the sealing film, a second sacrificial film havinga fourth opening portion smaller than an area of a region surrounded bythe support member in a region above the lower electrode, forming, onthe second sacrificial film, a layer thicker than a depth of the fourthopening portion so as to fill the fourth opening portion, forming atrench in a region, positioned on the support member, of the thicklayer, and forming a structure on the sealing film in the region aboveeach of the lower electrodes in one-to-one correspondence with the lowerelectrode, and removing the second sacrificial film via the trenchbetween adjacent structures, wherein a plurality of capacitance sensingelements comprising the lower electrode and upper electrode are formed.

In this fabrication method, a structure which includes an overhang and asupport portion supporting substantially the center of the overhang, andin which the area of the support portion is smaller than the area of theoverhang in the two-dimensional direction of the upper electrode isformed on the upper electrode in the region above each lower electrode.

In the above surface shape recognition sensor fabrication method, thefirst sacrificial film may be formed by hot-pressing, on thesemiconductor substrate, a dielectric material preformed on a base bycoating to bury the dielectric material in the support member and lowerelectrode, and peeling the base from the dielectric material, therebyforming a thin film having a flat surface on the semiconductorsubstrate, and exposing the support member by etching back the thinfilm.

In the above surface shape recognition sensor fabrication method, thestructure may be formed by forming a thin film by coating a bottom ofthe fourth opening portion and an upper surface of the secondsacrificial film with a photosensitive resin, forming the trench byremoving a portion of the thin film by exposure and development by usinga trench-shaped pattern, and thermally curing the thin film.

In the above surface shape recognition sensor fabrication method, thestep of forming the structure may comprise the steps of forming a secondmetal film on a bottom of the fourth opening portion and on the secondsacrificial film, forming a third mask pattern having an opening portionon the second metal film, forming a third metal pattern by plating on asurface of the second film exposed to a bottom of the third maskpattern, removing the third mask pattern to obtain a portion of thetrench, forming the trench by etching away the second metal film exposedto a bottom of the portion of the trench by using the third metalpattern as a mask, and etching away the second sacrificial film via thetrench.

The above surface shape recognition sensor fabrication method mayfurther comprise the steps of forming an etching stop film made of ametal on the sealing film, and forming the second sacrificial film onthe etching stop film by using a photosensitive material.

Another surface shape recognition sensor fabrication method according tothe present invention comprises the steps of forming an interleveldielectric on a semiconductor substrate, forming a first metal film onthe interlevel dielectric, forming a first metal pattern by plating on asurface of the first metal film exposed to a bottom of a first openingportion in the first mask pattern, removing the first mask pattern, andforming, on the first metal film and first metal pattern, a second maskpattern having a second opening portion around the first metal pattern,forming a second metal pattern thicker than the first metal pattern byplating on the surface of the first metal pattern exposed to a bottom ofthe second opening portion in the second mask pattern, removing thesecond mask pattern, and etching away the first metal film by using thefirst metal pattern and second metal pattern as masks, thereby forming alower electrode made of the first metal film and first metal pattern,and a support member made of the first metal film and second metalpattern, forming a first sacrificial film on the interlevel dielectricso as to cover the lower electrode and expose an upper portion of thesupport member, forming an upper electrode having a plurality of thirdopening portions on the first sacrificial film and support member,selectively removing the first sacrificial film via the third openingportions after the upper electrode is formed, forming a sealing film onthe upper electrode after the first sacrificial film is removed, forminga columnar pattern in a predetermined region on the upper electrode,laminating, on the columnar pattern, a photosensitive resin filmseparated from the sealing film, and forming a structure made up of thecolumnar pattern and thin film by removing a portion of the thin filminto a shape of a lattice, wherein a plurality of capacitance sensingelements comprising the lower electrode and upper electrode are formed.

The above surface shape recognition sensor fabrication method mayfurther comprise the step of adhering a thin rubber film on thestructure. The method can may further comprise the step of placing athin film on the structure, and fixing the thin film at an end of aregion in which the plurality of capacitance sensing elements areformed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic sectional view showing an example of thearrangement of a surface shape recognition sensor according to anembodiment of the present invention;

FIG. 1B is a schematic sectional view showing the example of thearrangement of the surface shape recognition sensor according to theembodiment of the present invention;

FIG. 1C is a perspective view showing the whole structure of the surfaceshape recognition sensor;

FIGS. 2A to 2P are views of steps for explaining an example of a methodof fabricating the surface shape recognition sensor according to theembodiment of the present invention;

FIGS. 3A to 3E are views of steps for explaining another example of themethod of fabricating the surface shape recognition sensor according tothe embodiment of the present invention;

FIGS. 4A to 4F are views of steps for explaining another example of themethod of fabricating the surface shape recognition sensor according tothe embodiment of the present invention;

FIGS. 5A to 5D are views of steps for explaining another example of themethod of fabricating the surface shape recognition sensor according tothe embodiment of the present invention;

FIG. 6 is a view of a step for explaining another example of the methodof fabricating the surface shape recognition sensor according to theembodiment of the present invention;

FIG. 7 is a view of a step for explaining another example of the methodof fabricating the surface shape recognition sensor according to theembodiment of the present invention;

FIG. 8 is a schematic sectional view showing another example of thearrangement of the surface shape recognition sensor according to theembodiment of the present invention;

FIG. 9A is a schematic sectional view showing another example of thearrangement of the surface shape recognition sensor according to theembodiment of the present invention;

FIG. 9B is a plan view schematically showing an example of thearrangement of a portion of the surface shape recognition sensoraccording to the embodiment of the present invention;

FIGS. 10A to 10I are views of steps for explaining another example ofthe method of fabricating the surface shape recognition sensor accordingto the embodiment of the present invention;

FIGS. 11A to 11F are views of steps for explaining another example ofthe method of fabricating the surface shape recognition sensor accordingto the embodiment of the present invention;

FIG. 12 is a graph for explaining the state of sensing by a conventionalsurface shape recognition sensor;

FIG. 13 is a graph for explaining the state of sensing by the surfaceshape recognition sensor according to the embodiment of the presentinvention;

FIG. 14 is a schematic sectional view showing an example of thearrangement of a conventional surface shape recognition sensor;

FIG. 15 is a schematic sectional view showing an example of thearrangement of a conventional surface shape recognition sensor; and

FIG. 16 is a schematic sectional view showing an example of thearrangement of a conventional surface shape recognition sensor.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawings.

First Embodiment

First, a surface shape recognition sensor according to the firstembodiment of the present invention will be described below withreference to FIGS. 1A, 1B, and 1C. FIGS. 1A and 1B are schematicsectional views showing an example of the arrangement of this sensor, inwhich FIG. 1A mainly illustrates a sensor element (sensor cell) 100 as aconstituent unit of the surface shape recognition sensor. For example, aplurality of sensor elements 100 are arranged in a matrix to form thesensing surface of this sensor. FIG. 1B shows the state in which afingertip as an object of surface shape sensing is in contact with thesensing surface of this sensor. Also, FIG. 1C is a perspective viewshowing the whole structure of the surface shape recognition sensor.

The arrangement of the sensor element 100 will be explained in detailbelow. On a substrate 101 made of single-crystal silicon or the like, alower electrode 105 a is formed via an interlevel dielectric 102. Thelower electrode 105 a is positioned in substantially the center of theregion of the sensor element 100. A support member 107 a is formedaround the lower electrode 105 a, and an upper electrode 110 b issupported on the support member 107 a.

The support member 107 a is, e.g., a lattice-shape member, and the lowerelectrode 105 a is formed in the center of each square of the lattice.Accordingly, each square of the lattice-shape support member 107 a isthe region of one sensor element 100. The upper electrode 110 b isintegrally formed on the support member 107 a, so one upper electrode110 b is formed for a plurality of lower electrodes 105 a. Also, theupper electrode 110 b is flexible, and a portion opposing the lowerelectrode 105 a is elastically deformable so as to deflect toward thelower electrode 105 a.

The upper electrode 110 b has a structure 113 b formed on it via asealing film 111. The structure 113 b is made up of a support portionwhich is in direct contact with the upper portion of the structure 113b, and an overhang which extends outward from this support portion. Thestructures 113 b adjacent to each other are separated from each other,and, for example, each structure 113 b is paired with the lowerelectrode 105 a. As is also shown in FIG. 1B, the structure 113 b isformed in each sensor element 100.

The overhang of the structure 113 b is formed so as not to exceed theregion of the sensor element 100 in the direction in which the upperelectrode 110 b runs. By contrast, the support portion of the structure113 b is made as small as possible so as to be able to support theoverhang.

The overhang of the structure 113 b need only have a substantiallysquare planar shape. For example, when the sensor elements 100 arearranged at an interval of 50 μm, it is only necessary to form theoverhang of the structure 113 b as a square of 45 μm side, and form thesupport portion in contact with the sealing film 111 as a square of 5 μmside.

In this surface shape recognition sensor, as shown in FIG. 1B, the upperelectrode 110 b below the structure 113 b which is pushed down whentouched by a finger 1602 deflects downward, and this changes thecapacitance formed by the upper electrode 110 b and lower electrode 105a. In this state, a downward pushing force is applied to the overhanghaving a large area of the structure 113 b, and this force istransmitted to the upper electrode 110 b via the support portion havinga small area of the structure 113 b. Accordingly, a force applied perunit area increases in the upper electrode 110 b.

In this embodiment as described above, the force applied tosubstantially the entire area of the sensor element 100 when the finger1602 touches is concentrated to the support portion of the structure 113b and transmitted to the upper electrode 110 b. In this embodiment,therefore, the sensitivity of the surface shape recognition sensor canbe increased. Also, in this embodiment, the spacing between the adjacentstructures 113 b is narrow, so it is possible to prevent the structure113 b from sinking into the finger 1602 to disperse the force.

The operation of the this surface shape recognition sensor will bebriefly explained below. When a finger touches the sensing surface onwhich the plurality of structures 113 b are arranged, and pushes onestructure 113 b downward, the upper electrode 110 b of the sensorelement 100 below the structure 113 b that is pushed down deflectsdownward. This deflection changes the capacitance formed between theupper electrode 110 b and lower electrode. The shape of the fingerprintcan be regenerated by forming continuous tone data in accordance withthose changes in capacitances formed on the lower electrodes 105 a,which correspond to the fingerprint shape.

Note that the sensing of the capacitance in each sensor cell when theupper electrode 110 b deforms and the conversion into the continuoustone data are performed by, e.g., an integrated circuit (not shown)formed on the substrate 101. Note also that when, for example, the upperelectrode 110 b is grounded, static electricity generated in an objectflows to ground even if this static electricity is discharged to theupper electrode 110 b. By thus connecting the upper electrode 110 b toground, the integrated circuit can be protected from electrostaticdestruction.

A method of fabricating the surface shape recognition sensor accordingto this embodiment described above will be explained below. First, asshown in FIG. 2A, an interlevel dielectric 102 made of silicon nitrideor the like is formed on a substrate 101 made of a semiconductormaterial such as silicon. Although not shown, an integrated circuit suchas a sensing circuit and an interconnecting structure having a pluralityof interconnections are formed on the substrate 101 below the interleveldielectric 102. After the interlevel dielectric 102 is formed, a seedlayer (first metal film) 103 which is a two-layered film made up of a0.1-μm thick titanium film and 0.1-μm thick gold film is formed by vapordeposition or the like. Note that the interlevel dielectric 102 may alsobe made of silicon oxide, but it is preferably made of silicon nitridewhen an HF-based etching process (to be described later) is taken intoconsideration.

Subsequently, as shown in FIG. 2B, a resist pattern (first mask pattern)104 about 5 μm thick having an opening portion 104 a is formed on theseed layer 103. The resist pattern 104 is formed by the well-knownphotolithography technique. When the resist pattern 104 is formed, ametal pattern (first metal pattern) 105 about 1 μm thick made of a goldplating film is formed by electroplating on the seed layer 103 exposedto the opening portion 104 a. Since the film is formed byelectroplating, the gold plating film is not formed on the resistpattern 104, and the metal pattern 105 is selectively formed on the seedlayer 103 exposed to the opening portion 104 a.

After the resist pattern 104 is removed, as shown in FIG. 2C, a 5-μmthick resist pattern (second mask pattern) 106 having an opening portion106 a is formed. The resist pattern 106 is formed so that it covers themetal pattern 105 and the opening portion 106 a is positioned in aprospective region of the support member 107 a shown in FIG. 1. Afterthe resist pattern 106 is formed, a metal pattern (second metal pattern)107 about 3 μm thick made of a gold plating film is formed byelectroplating on the seed layer 103 exposed to the opening portion 106a.

After the resist pattern 106 is removed, the metal patterns 105 and 107are used as masks to selectively etch the seed layer 103. In thisetching, an etchant containing iodine, ammonium iodide, water, andethanol is first used to selectively remove the gold as the upper layerof the seed layer 103. Then, an HF-based etchant is used to selectivelyremove the titanium as the lower layer of the seed layer 103. Note thatthe etching rate in the gold wet etching is 0.05 μm/min.

Consequently, as shown in FIG. 2E, a lower electrode 105 a having thegold upper layer and a support member 107 a insulated and separated fromthe lower electrode 105 a are formed on the substrate 101. As shown inFIG. 1, the support member 107 a supports the upper electrode 110 b.Also, as shown in the plan view of FIG. 2D, the support member 107 a isformed into the shape of a lattice on the substrate 101. A plurality oflower electrodes 105 a are arranged in the centers of regions surroundedby the lattice-shaped support member 107 a.

When the lower electrode 105 a and support member 107 a are formed asdescribed above, as shown in FIG. 2E, a sacrificial film 108 (firstsacrificial film) is so formed as to cover the lower electrode 105 a andexpose the upper surface of the support member 107 a. The formation ofthe sacrificial film 108 will be briefly explained below. First, aphotosensitive resin film is formed on the substrate 101 by spincoating, thereby covering the lower electrode 105 a and support member107 a. The resin film has positive photosensitivity, and is formed by,e.g., adding a positive photosensitive agent to a base resin (polyimide)such as polyamide, polyamide acid, or polybenzoxazole (or itsprecursor).

After a heating process (prebaking) is performed, the region above thesupport member 107 a is exposed by using the well-known photolithographytechnique, and development is subsequently performed to expose the upperportion of the support member 107 a. After that, the resin film is curedby a heating process and etched back by chemical mechanical polishing,thereby substantially leveling the support member 107 a and sacrificialfilm 108 with each other as shown in FIG. 2E.

After the sacrificial film 108 is formed as described above, as shown inFIG. 2F, on the sacrificial film 108 in which the upper surface of thesupport member 107 a is exposed by planarization, a seed layer 109 whichis a two-layered film made up of a 0.1-μm thick titanium film and 0.1-μmthick gold film is formed by vapor deposition or the like.

Then, as shown in FIG. 2G, a columnar resist pattern 201 is formed onthe seed layer 109, and a metal film 110 about 1.0 μm thick made of agold plating film is formed by electroplating on the exposed seed layer109 in a region where the resist pattern 201 is not formed.

After the resist pattern 201 is removed, the formed metal film 110 isused as a mask to selectively etch away the seed layer 109.Consequently, as shown in FIG. 2H, an upper electrode 110 b having aplurality of opening portions 110 a is formed. The opening portions 110a have a square planar shape of 4 μm side, and are arranged at the fourcorners inside the support member 107 a.

Subsequently, as shown in FIG. 2I, the sacrificial film 108 is removedvia the opening portions 110 a in the upper electrode 110 b to form aspace below the upper electrode 110 b. With the formed space sandwichedbetween them, the upper surface of the lower electrode 105 a opposes thelower surface of the upper electrode 110 at a predetermined distance.Note that the upper electrode 110 b is supported by the support member107 a. Note also that the sacrificial film 108 is removed by exposingthe substrate 101 to a plasma mainly containing oxygen gas, and bringingthe etching species generated by the plasma into contact with thesacrificial film 108 via the opening portions 110 a.

As shown in FIG. 2J, a sealing film 111 is laminated and adhered to theupper electrode 110 b to close the opening portions 110 a while thespace between the upper electrode 110 b and lower electrode 105 a isheld. The formation of the sealing film 111 by lamination was performedby using an STP technique (Spin-coating film Transfer and hot-Pressingtechnique). This technique will be briefly explained below. First, aresin film preformed on a sheet film by coating is hot-pressed on theupper electrode 110 b in a vacuum. Then, the sheet film is peeled fromthe resin film, and the resin film laminated on the upper electrode 110b is cured by a heat treatment which holds the temperature at 300° C.for about 1 hr. As a consequence, the sealing film 111 made of the resinfilm is formed on the upper electrode 110 b. The resin film is made of abase resin (polyimide) such as polyamide, polyamide acid, orpolybenzoxazole (or its precursor).

After the sealing film 111 is formed as described above, as shown inFIG. 2K, a silicon oxide film 112 about 1.5 μm thick is deposited on thesealing film 111 by sputtering. Then, a resist pattern about 3 μm thickhaving a square opening portion of about 5 μm side in substantially thecenter of the upper electrode 10 b is formed by the well-knowphotolithography technique. The silicon oxide film 112 on the bottom ofthe opening portion in this resist pattern is etched away by ah HF-basedetchant, and the resist pattern is removed after that. Consequently, asshown in FIG. 2L, a sacrificial film 112 b (second sacrificial film)made of the silicon oxide film and having an opening portion 112 a isformed.

As will be explained below, the opening portion 112 a in the sacrificialfilm 112 b is a mold for forming a support portion for supporting thestructure 113 b. Therefore, the opening portion 112 a is formed to havean area smaller than at least the element region surrounded by thesupport member 107 a, preferably, smaller than the lower electrode 105a. Also, the opening portion 112 a is positioned in the region abovesubstantially the center of the lower electrode 105 a. The depth of theopening portion 112 a is the height of the support portion of thestructure 113 b. Accordingly, the thickness of the sacrificial film 112b is appropriately set in accordance with the shape of the desiredstructure 113 b.

As shown in FIG. 2M, a photosensitive resin film 113 about 10 μm thickis formed by spin coating on the sacrificial film 112 b and sealing film111. The resin film has positive photosensitivity, and is formed by,e.g., adding a positive photosensitive agent to a base resin (polyimide)such as polyamide, polyamide acid, or polybenzoxazole (or itsprecursor).

After the resin film 113 is formed by coating and heated (prebaked), aregion substantially corresponding to the upper portion of the supportmember 107 a is exposed by the well-known photolithography technique. Asshown in FIG. 2N, development is subsequently performed to form trenches113 a and a structure 113 b. The width of the trench 113 a is about 5μm. After that, the structure 113 b is cured by a heating process bywhich it is held at about 300° C. for 1 hr.

The sacrificial film 112 b is then etched away by allowing an HF-basedetchant to act from the trenches 113 a. Consequently, as shown in FIG.20, the structure 113 b is given a support portion which is in directcontact with the upper surface of the sealing film 111, and an overhangwhich extends outward from this support portion. The structure 113 b hasa T-shaped section, and the area of the uppermost portion is larger thanthe portion adhered to the sealing film 111. Also, the structures 113 bare formed in one-to-one correspondence with the lower electrodes 105 a,and the overhangs of adjacent structures 113 b are separated by about 5μm.

In this embodiment, the lower portion (support portion) of the structure113 b is narrower than its upper portion (overhang), and adhered tosubstantially the center of the upper electrode 110 b. Also, thethickness of the upper electrode 110 b positioned immediately below theadhered portion between the structure 113 b and sealing film 111apparently increases by the presence of the structure 113 b, so theupper electrode 110 b does not easily deflect. However, the upperelectrode 110 b around the adhered portion has the thickness of theupper electrode 110 b and sealing film 111, and easily deflects.

The adhered portion between the structure 113 b and sealing film 111 ispositioned in the center of the upper electrode 110 b, and thedeflection of the upper electrode 110 b is a maximum in this portion.Therefore, the structure 113 b efficiently transmits a force receivedfrom a finger to the upper electrode 110 b, and this makes it possibleto increase the deflection of the upper electrode 110 b.

Note that in the above embodiment a silicon oxide film is used as thesecond sacrificial film (sacrificial film 112 b) and an HF-based etchantis used as the etchant for the film, but the present invention is notlimited to this embodiment. It is also possible to from the secondsacrificial film from titanium, and remove the film by an HF-basedetchant. Alternatively, it is possible to form the second sacrificialfilm from a copper film obtained by plating, and remove the film by anetchant containing nitric acid or the like.

Furthermore, the sacrificial film 108 is formed by forming aphotosensitive resin film by spin coating and forming a flat surface byetching back the film by chemical mechanical polishing, but it is alsopossible to form a flat surface by filling the support member 107 a andlower electrode 105 a with a photosensitive film by the STP techniquewithout using any chemical mechanical polishing, and exposing thesupport member 107 a by etching back as needed.

In addition to the above structure, as shown in FIG. 2P, a thin film 120may also be formed on the structure 113 b. As the thin film 120, a resinfilm about 2 μm thick, for example, can be adhered on the upper surfaceof the structure 113 b by using the STP technique. It is also possibleto adhere rubber about 10 μm thick on the upper surface of the structure113 b. This prevents a portion of an object from entering the trenches113 a around the structure 113 b. Also, since the film is made ofrubber, the structures 113 b adjacent to each other do not completelyinterlock with each other, but can move independently of each other.

Alternatively, a film about 10 μm thick made of an organic material canbe formed as the thin film 120, and fixed at the end portions of thesensing surface on which the plurality of structures 113 b are arranged.In this structure, the trenches 113 a are completely closed from theoutside, and the film is not adhered to the upper surface of thestructure 113 b, so the structures 113 b adjacent to each other can moveindependently of each other.

Second Embodiment

Another embodiment of the present invention will be described below.

First, following the same procedures as in FIGS. 2A to 2J, a lowerelectrode 105 a, upper electrode 110 b, and the like are formed, and theupper electrode 110 b is covered with a sealing film 111.

Subsequently, a silicon oxide film is formed on the sealing film 111 bysputtering or the like, and patterned by the well-know photolithographytechnique or the like, thereby forming a sacrificial film 301 (secondsacrificial film) having an opening portion 301 a as shown in FIG. 3A.

After the sacrificial film 301 is formed as described above, as shown inFIG. 3B, a seed layer (second metal film) 302 which is a two-layeredfilm including a 0.1-μm thick chromium film and 0.1-μm thick gold filmis formed by vapor deposition or the like. Then, as shown in FIG. 3C, aresist pattern 303 (third mask pattern) is formed. The resist pattern303 is a lattice-shaped pattern, and formed in a region where a supportmember 107 a is formed.

In addition, to fill the squares of the resist pattern 303 halfway, ametal pattern (third metal pattern) 304 about 10 μm thick is formed onthe seed layer 302. The metal pattern 304 need only be selectivelyformed on the exposed seed layer 302 by electroplating.

When the resist pattern 303 is removed after the metal pattern 304 isformed as described above, a metal pattern 304 having opening portions304 a is formed as shown in FIG. 3D.

After that, the metal pattern 304 is used as a mask to selectively etchthe seed layer 302 exposed to the bottoms of the opening portions 304 a.In this etching, an etchant containing iodine, ammonium iodide, water,and ethanol is first used to selectively remove the gold as the upperlayer of the seed layer 302. Then, an etchant containing potassiumferricyanide and sodium hydroxide is used to selectively remove thechromium as the lower layer of the seed layer 302 (FIG. 3D).

Subsequently, the sacrificial film 301 is etched away via the openingportions 304 a. In this etching, an HF-based etchant was used.Consequently, as shown in FIG. 3E, a structure made of the metal pattern304 and having an upper portion larger than a lower portion is formed.

In this embodiment, a silicon oxide film is used as the secondsacrificial film, and an HF-based etchant is used as the etchant forthis film, but the present invention is not limited to this embodiment.For example, it is also possible to form the second sacrificial filmfrom titanium, and remove the film by using an HF-based etchant.

The following method is also possible. A 0.2-μm thick titanium film 401(etching stop film) is formed on the sealing film 111 by vapordeposition (FIG. 4A). After that, a second sacrificial film 402 made ofa resin film is formed by coating, and an opening portion 402 a isformed by the well-known photolithography technique. The sacrificialfilm 402 has positive photosensitivity, and is formed by, e.g., adding apositive photosensitive agent to a base resin (polyimide) such aspolyamide, polyamide acid, or polybenzoxazole (or its precursor).

Then, a seed layer 403 (second metal film) made of a two-layered filmincluding a 0.1-μm thick titanium film and 0.1-μm thick gold film isformed by vapor deposition or the like (FIG. 4B). As shown in FIG. 4C, aframe-shaped resist pattern 404 is formed and used as a mask pattern toform a gold plating film about 10 μm thick on the seed layer 403 byelectroplating. When the resist pattern 404 is removed after that, asshown in FIG. 4D, a metal pattern 405 (third metal pattern) havingopening portions in portions from which the resist pattern 404 isremoved is formed.

After that, the metal pattern 405 is used as a mask to selectively etchaway the seed layer 403. The substrate 101 is then exposed to a plasmamainly containing oxygen gas, and the etching species generated by theplasma is brought into contact with the sacrificial film 402, therebyremoving the sacrificial film 402 (FIG. 4E). In this manner, a metalpattern 405 which is a structure having an upper portion larger than alower portion is formed.

If necessary, as shown in FIG. 4F, it is also possible to remove aportion of the titanium film 401 on the sealing film 111 or to removethe titanium film as the lower layer of the seed layer 403 by using anHF-based etchant, thereby exposing the surface of the sealing film 111.

Third Embodiment

Another embodiment of the present invention will be described below.

First, following the same procedures as in FIGS. 2A to 2J, a lowerelectrode 105 a, upper electrode 110 b, and the like are formed, and theupper electrode 110 b is covered with a sealing film 111 (FIG. 5A).

Subsequently, as shown in FIG. 5B, a structure lower portion 501(columnar pattern) is formed by a resin film on the sealing film 111.The structure lower portion 501 has a substantially square planar shapeof about 5 μm side. The resin film has positive photosensitivity, and isformed by, e.g., adding a positive photosensitive agent to a base resin(polyimide) such as polyamide, polyamide acid, or polybenzoxazole (orits precursor).

After this resin film about 5 μm thick is formed on the sealing film 111by spin coating, as shown in FIG. 5B, the well-known photolithographytechnique is used to remove the resin film by exposure and developmentfrom a region except for a square region of about 5 μm side on a centralportion of the upper electrode 110 b. After that, the resin film iscured by performing a heat treatment at about 300° C. for 1 hr, therebyforming a structure lower portion 501 made of the resin film.

Then, a resin film 502 is laminated by hot-pressing by using the STPtechnique. The resin film 502 has positive photosensitivity, and isformed by, e.g., adding a positive photosensitive agent to a base resin(polyimide) such as polyamide, polyamide acid, or polybenzoxazole (orits precursor). In this lamination, the resin film formed on a sheetfilm by coating is hot-pressed and laminated on the structure lowerportion 501 such that the resin film comes in contact with the structurelower portion 501 and does not contact the sealing film 111. After that,as shown in FIG. 5C, only the sheet film is peeled off to form a resinfilm 502.

After the resin film 502 is formed as described above, the resin film502 is patterned using the well-known photolithography technique. Inthis patterning, the resin film 502 on the upper perimeter of a supportmember 107 a is exposed by a frame-shaped pattern and developed. By thispatterning, as shown in FIG. 5D, opening portions 502 a and a structureupper portion 502 b are formed. After that, the structure upper portion502 b having the opening portions 502 a is cured by a heating processwhich holds the temperature at about 300° C. for 1 hr, thereby forming astructure 503 made up of the structure lower portion 501 and structureupper portion 502 b.

In this embodiment explained with reference to FIGS. 5A to 5D, the STPtechnique is used to form the structure having the upper portion largerthan the lower portion. Therefore, this embodiment eliminates the stepof forming and etching away a sacrificial film, and makes it possible toreduce the number of steps of fabricating the surface shape recognitionsensor.

In this embodiment, as shown in FIG. 5A, a film having a uniform filmthickness is used as the sealing film 111, but the present invention isnot limited to this embodiment. For example, as shown in FIG. 6, it isalso possible to use a sealing film 601 integrated with a projection 601a which corresponds to the structure lower portion.

The sealing film 601 can be formed as follows. First, a 6-μm thick resinfilm is laminated and adhered to the upper electrode 110 b by the STPtechnique, thereby closing the opening portions 110 a. After that, asshown in FIG. 6, a region except for a portion corresponding to thestructure lower portion is thinned by exposure and development, therebyforming the sealing film 601 having the projection 601 a.

Although in FIG. 5D the resin film 502 is formed in contact with theupper surface of the structure lower portion 501, the present inventionis not limited to this arrangement.

For example, as shown in FIG. 7, the upper portion of the projection 601a of the sealing film 601 can also fit in the lower portion of an upperstructure 701, provided that the lower surface of the upper structure701 is not in contact with a region except for the lower structure ofthe sealing film 601.

In the above description, the support portion forming the lower portionof the structure is made up of one member, but the present invention isnot limited to this arrangement. For example, as shown in FIG. 8, thesupport portion of a structure 801 may also be made up of a plurality ofcolumns 802. Note that the rest of the arrangement shown in FIG. 8 isthe same as that shown in FIGS. 1A to 1C, so an explanation thereof willbe omitted.

Also, the overhang as the upper portion of the structure is supported bythe support portion formed in substantially the center of the overhangas described above, but the present invention is not limited to thisarrangement. As shown in FIGS. 9A and 9B, perimeter support portions 902made of an elastic material can be formed at the four corners of theoverhang having a rectangular planar shape of a structure 901 supportedby a support portion 901 a formed in substantially the center. Theperimeter support portions 902 can be made of, e.g., rubber or coilsprings. The perimeter support portions 902 can prevent a fall of thestructure 901.

When the structure 901 is pushed down toward the lower electrode 105 a,the perimeter support portions 902 made of an elastic materialelastically deform and crush, and the sealing film 111 and upperelectrode 110 b are pushed down by the support portion 901 a formed inthe center. Even when the perimeter support portions 902 are formed asdescribed above, the transmission of the force to the upper electrode110 b by the structure 901 is not inhibited because the perimetersupport portions 902 are made of an elastic material.

An arrangement in which the support portion as the structure lowerportion and the overhang as the structure upper portion are made ofdifferent materials will be explained below.

First, following the same procedures as in FIGS. 2A to 2J, a lowerelectrode 105 a, upper electrode 110 b, and the like are formed, and theupper electrode 110 b is covered with a sealing film 111. Subsequently,a seed layer 1001 made of a two-layered film including a 0.1-μm thicktitanium film and 0.1-μm thick copper film is formed on the sealing film111 by vapor deposition, sputtering, or the like, and the well-knownphotolithography technique is used to form a resist pattern 1002 foreach sensor element (FIG. 10A). The resist pattern 1002 is a patternhaving a rectangular planar shape formed in a portion as a structurelower portion (to be described below).

As shown in FIG. 10B, copper is deposited by electroplating on the seedlayer 1001 exposed around the resist pattern 1002, thereby forming acopper sacrificial film 1003.

The resist pattern 1002 is then removed, and the sacrificial film 1003is used as a mask to remove the seed layer 1001 below the resist pattern1002, thereby exposing the upper surface of the sealing film 111, abovethe lower electrode 105 a, to an opening portion 1003 a as shown in FIG.10C.

Then, a photosensitive polyimide film is formed on the sacrificial film103 and patterned by the photolithography technique, thereby forming astructure lower portion 1004 made of polyimide (resin) as shown in FIG.10D.

As shown in FIG. 10E, a seed layer 1005 is so formed as to cover thesurfaces of the structure lower portion 1004 and sacrificial film 1003,and a resist pattern 1006 is subsequently formed. The seed layer 1005 isa two-layered film including an upper layer made of a 0.1-μm titaniumfilm and a lower layer made of a 0.1-μm thick gold film. Also, theresist pattern 1006 is a lattice-shaped pattern, and formed in a regionwhere a support member 107 a is formed.

As shown in FIG. 10F, a gold film 1007 about 5 μm thick is formed on theseed layer 1005 so as to fill the squares of the resist pattern 1006halfway. The gold film 1007 is formed on the exposed seed layer 1005 byplating gold by electroplating.

After that, as shown in FIG. 10G, the resist pattern 1006 is removed toform a structure upper portion 1008 made of gold in each sensor element.

As shown in FIG. 10H, the structure upper portion 1008 is used as a maskto remove the seed layer 1005 by wet etching. For example, the gold asthe upper layer of the seed layer 1005 can be etched by using, e.g., anetchant containing iodine, ammonium iodide, water, and ethanol. Also,the titanium as the lower layer of the seed layer 1005 can be etched byusing an HF-based etchant. Consequently, the upper surface of thesacrificial film 1003 is exposed around the structure upper portion1008.

Finally, as shown in FIG. 10I, in the region exposed around thestructure upper portion 1008, the sacrificial film 1003 made of copperis etched by an etchant containing nitric acid, and the titanium as thelower layer of the seed layer 1001 is etched by an HF-based etchant,thereby forming a structure made up of the structure lower portion 1004made of polyimide and the structure upper portion 1008 made of gold.This structure has an overhang as the structure upper portion 1008, andthe structure lower portion 1004 which supports substantially the centerof the structure upper portion 1008. The structure is formed in theposition of the lower electrode 105 a in each sensor chip.

In the surface shape recognition sensor shown in FIG. 10I, the overhangof the structure formed on the upper electrode 110 b is made of a metal.Therefore, the overhang is a rigid body having a high Young's modulus,does not easily deform, and hardly interferes with deformation of theupper electrode when a fingerprint shape is sensed. Also, when comparedto a case in which the overhang is made of a synthetic resin, therigidity can be ensured even if the thickness is decreased, so thesurface shape recognition sensor shown in FIG. 10I can be fabricatedwithin a shorter time period.

Another example of the arrangement in which the support portion as thestructure lower portion and the overhang as the structure upper portionare made of different materials will be explained below.

Following the same procedures as in FIGS. 2A to 2J, a lower electrode105 a, upper electrode 110 b, and the like are formed, and the upperelectrode 110 b is covered with a sealing film 111. Subsequently, a0.1-μm thick adhesive layer 1101 made of titanium is formed on thesealing film 111 by, e.g., vapor deposition or sputtering, a polyimidelayer 1102 is formed on the adhesive layer 1101, and a seed layer 1103is formed on the polyimide layer 1102 (FIG. 11A). The polyimide layer1102 can be formed by spin coating and thermal cure of a polyimideresin. Also, the seed layer 1103 is a two-layered film including a lowerlayer made of a 0.1-μm thick titanium film, and an upper layer made of a0.1-μm thick gold film. In the seed layer 1103, the titanium lower layerimproves the adhesion to the polyimide layer 1102.

Subsequently, as shown in FIG. 11B, a resist pattern 1104 is formed onthe seed layer 1103 by the well-known photolithography technique. Theresist pattern 1104 is a lattice-shaped pattern, and formed in a regionwhere a support member 107 a is formed.

As shown in FIG. 1C, a gold film 1105 about 5 μm thick is formed on theseed layer 1103 so as to fill the square of the resist pattern 1104halfway. The 5-μm thick gold film 1105 is formed on the exposed seedlayer 1103 by plating gold by electroplating.

After that, as shown in FIG. 1D, the resist pattern 1104 is removed toform a structure upper portion 1106 made of gold in each sensor element.Also, the seed layer 1103 is removed by wet etching by using thestructure upper portion 1106 as a mask. For example, the gold as theupper layer of the seed layer 1103 can be etched by using an etchantcontaining iodine, ammonium iodide, water, and ethanol. Also, thetitanium as the lower layer of the seed layer 1103 can be etched byusing an HF-based etchant. Consequently, the upper surface of thepolyimide layer 1102 is exposed around the structure upper portion 1106.

Finally, as shown in FIG. 1E, in the region exposed around the structureupper portion 1106, a predetermined amount of the polyimide layer 1102is etched by dry etching using oxygen plasma, thereby forming astructure lower portion 1107 made of polyimide. In this state, the lowersurface of the structure lower portion 1107 is connected and fixed onthe sealing film 111 via the adhesive layer 1101, and the upper surfaceof the structure lower portion 1107 is connected and fixed to thestructure upper portion 1106 via the seed layer 1103.

Note that as shown in FIG. 11F, it is also possible to remove portionsother than the adhesive layer 1101 and seed layer 1103 in contact withthe structure lower portion 1107 by wet etching.

In the present invention as has been explained above, a structure whichis made up of an overhang and a support portion supporting substantiallythe center of the overhang, and in which the area of the support portionis smaller than the area of the overhang in the two-dimensionaldirection of an upper electrode is formed on the upper electrode in aregion above each lower electrode. In a surface shape recognition sensorhaving this arrangement, an object of surface shape sensing, e.g., afingertip touches the surface of the overhang of the structure, and thesupport portion of this structure whose overhang is in contact with theobject of sensing pushes down a portion of the upper electrode towardthe lower electrode, thereby deforming the upper electrode.

This structure can efficiently transmit the force received from thefinger to the upper electrode, increase the deflection of the upperelectrode, and thereby increase the sensing sensitivity of this sensor.

For example, as shown in FIG. 12, in the conventional structure havingno overhang, if the surface of a finger as an object of fingerprintsensing is soft, the output from the sensor does not largely increaseeven when the force of pushing the finger increases. By contrast, in thearrangement of the present invention having the overhang, even if thesurface of a finger is soft, the sensor output equivalent to thatobtained by a hard finger can be obtained by increasing the force whichpushes the fingertip.

Also, in the arrangement of the present invention, the spacing betweenadjacent structures is narrow, and the upper surface of each structureis a substantially flat surface, so a remarkable effect is obtained bywhich the sensor does not easily break even if a force is appliedsideways by an object of sensing such as a finger, i.e., by which thehigh mechanical strength is high.

As described above, the surface shape recognition sensor according tothe present invention is suited to sensing fingerprints at highaccuracy.

1. A surface shape recognition sensor characterized by comprising: aplurality of capacitance sensing elements comprising a plurality oflower electrodes arranged in the same plane on a substrate so as to beinsulated and separated from each other, and a deformable plate-shapedupper electrode formed above said lower electrodes with a predeterminedspacing therebetween, and made of a metal; a support member which isformed around said lower electrode so as to be insulated and separatedfrom said lower electrode, and supports said upper electrode; and astructure formed on said upper electrode in a region above each of saidlower electrodes in one-to-one correspondence with said lower electrode,wherein said structure comprises an overhang and a support portion whichsupports substantially a center of said overhang, and an area of saidsupport portion is smaller than an area of said overhang in atwo-dimensional direction of said upper electrode.
 2. A surface shaperecognition sensor according to claim 1, characterized by furthercomprising a thin flexible film formed on said overhang of saidstructure, and extending over a plurality of said structures.
 3. Asurface shape recognition sensor according to claim 1, characterized byfurther comprising a perimeter support portion formed below a perimeterof said overhang, and made of an elastic material.
 4. A surface shaperecognition sensor according to claim 1, characterized in that saidsubstrate is a semiconductor substrate on which an integrated circuit isformed, said lower electrode is placed on a interlevel dielectric formedon said semiconductor substrate, and said integrated circuit includes asensing circuit which senses a capacitance formed on said lowerelectrode.